Preheat your oven to 350°F (175°C) and prepare a 9-inch round cake pan by greasing it with butter or lining it with parchment paper.
In a large mixing bowl, mash the ripe bananas until smooth. You can use a fork or a potato masher for this step.
Add the softened butter to the mashed bananas and mix until well combined. The mixture should be creamy and free of lumps.
In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set aside.
In another mixing bowl, cream together the granulated sugar and eggs until light and fluffy. This can be done using an electric mixer or by hand using a whisk.
Add the vanilla extract to the sugar and egg mixture and mix well.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Start and end with the dry ingredients, mixing each addition until just combined. Be careful not to overmix as this can result in a dense cake.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake in the preheated oven for about 35-40 minutes or until a toothpick inserted into the center of the cake comes out clean.
Once baked, remove the cake from the oven and let it cool in the pan for 10 minutes. Then transfer it to a wire rack to cool completely.